| Coal gangue is an industrial by product discharged during the process of coal exploitation and processing.Our energy consumption structure is dominated by coal,and coal gangue production is increasing yearly,which causes serious environmental pollution problems.As a kind of industrial solid waste,limited utilization of coal gangue and higher processing cost is one of the bottlenecks restricting the development of green transformation in China.Studying the preparation technology of high-value ceramic materials using related solid wastes is urgent.The potential of high value utilization of coal gangue was fully explored based on the characteristics of valuable minerals,high contents of aluminum and silicon oxide.The preparation of densiity and high strength mullite composite ceramics material using coal gangue as the main raw materials and the solution to the problem of harmful effect of high temperature liquid on the high temperature performance of mullite composite ceramics were studied in this dissertation,which has positive significance to expand the way of high value and large-scale utilization of coal gangue.The main results of the dissertation are as follows:（1）The liquid-solid uniform nucleation and growth mechanism of mullite was studied by using gangue and KAl（SO4）2·12H2O as raw materials,and using the liquid phase environment formed by K2SO4 and Na Cl composite molten salts.By controlling the critical reaction variables,mullite powders were prepared.The results show good crystallinity and uniform morphology of mullite can be obtained,when the mass ratio of coal gangue to composite molten salt is 1:2 and calcined at 850℃for 3 h,the mullite nanopowder with the particle size of 20-50 nm.Compared with the traditional methods,the temperature is reduced by 200～300°C.（2）Mullite composite ceramics were prepared by solid phase reaction sintering using mullite powder prepared by molten salt method and different aluminum sources.The effects of Al（OH）3 addition on the phase reconstruction,structural evolution and mechanical properties of mullite composite ceramics were studied.The results show that the addition of Al（OH）3 increased the contents of the mullite phase,and the samples sintered densely with the increase in sintering temperature.The composite ceramics exhibit optimal performances with the addition of 10 wt%Al（OH）3 at 1560°C,a bulk density of 2.43 g/cm3 and a flexural strength of 124.28 MPa,respectively.The increase in mechanical strength was mainly due to the bridging effect of needle and rod mullite and corundum formed by reaction sintering,and the corundum particle was"pinned"within the grain boundary of the mullite interlocking structure.These results can increase the path inside the material to hinder the crack growth,producing a synergistic strengthening effect and improving the material properties.（3）Based on the good properties of mullite composite ceramics,the effect of adding Al2O3 on the properties of mullite composite ceramics was studied by replacing Al（OH）3with Al2O3.The results show that the bulk density and bending strength of the samples increased greatly with the temperature increasing from 1480℃to 1580℃.The microstructure of the samples showed a three-dimensional network structure,which gave the composite ceramics excellent mechanical properties.The maximum bulk density,flexural strength,and fracture toughness for the sample with 15 wt%Al2O3 content are2.48 g/cm3,139.79 MPa,and 5.62 MPa?m1/2,respectively,with the temperature of 1560℃for 3 h.The strength and toughness of mullite composite ceramics were improved above the reported results.（4）To explore the high-value utilization of coal gangue,the effect of Fe2O3 addition on the structure and reaction mechanism of mullite composite ceramics prepared from coal gangue were studied.The results showed that when Fe2O3 was added,the Al2O3-Si O2 binary system transformed into Fe2On-Al2O3-Si O2 ternary system,and solid-solid sintering was replaced by liquid-solid sintering.The iron cations replaced and occupied the tetrahedral or octahedral site of the aluminum cations in the mullite lattice,development and growth of mullite embryos,and the mullite was completely at 1500℃.Negatively charged vacancies are left in the original lattice position,which further provides channels for iron cations diffusion and migration,and accelerates the crystallization growth of mullite.Meanwhile,rod-shaped mullite grains with an aspect ratio of～9 were formed.It was found that 6 wt%Fe2O3 was optimal for the formation of rod-shaped mullite after sintering at 1550°C for 3 h.The sample’s maximum bulk density was 2.84 g/cm3,with a flexural strength of 112 MPa.A dense network structure was developed,thus leading to mullite ceramics with excellent mechanical properties.（5）High-performance mullite-based composite ceramics are successfully prepared using natural kaolin and alumina as raw materials and Zr O2 as an additive.The influence of sintering temperature and Zr O2 content on sintering behavior and mechanical properties of composite ceramics is systematically studied.The results indicate that with the increase in sintering temperature from 1450 to 1560°C,the primary phases of as-sintered composite ceramics become mullite and corundum with a small amount of Zr O2.The bulk density of the composite ceramics increased from 2.29 to 2.72 g/cm3.Furthermore,the Zr O2 phase transition promoted the transgranular fracture and Zr O2grains were pinned at the grain boundaries,thereby enhancing the mechanical strength of the composite ceramics.Moreover,the AZS12 sample,with 12 wt%Zr O2 and sintered at 1560°C,renders the highest flexural strength and fracture toughness of 91.6 MPa and2.47 MPa.m1/2,respectively.The addition of Zr O2 increased the flexural strength by about37.6%due to the synergistic effect of zirconia phase transformation and Zr O2 pinning at grain boundaries.These results provide valuable insights into preparing high-performance mullite-based composite ceramics using low-cost kaolin as raw materials.Figure[80]Table[20]Reference[242]... |
PDF Full Text Request